Method for repairing an airfoil surface having an elastomeric protective coating

ABSTRACT

This invention relates to the repair and removal of erosion or impact damage using hand sandable elastomeric coatings on a curved substrate, particularly such surfaces as the leading edge of the airfoil. Specialized applicators and methods of use are also disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a divisional of and claims priority to U.S.patent application Ser. No. 13/308,788, filed on Dec. 1, 2011, which isa divisional of and claims priority under 35 U.S.C. § 120 of U.S. patentapplication Ser. No. 11/640,050, filed on Dec. 14, 2006, issued as U.S.Pat. No. 8,091,227 on Jan. 10, 2012, which claimed priority under 35U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No.60/750,536, filed on Dec. 14, 2005. Each patent application identifiedabove is incorporated here by reference in its entirety to providecontinuity of disclosure.

FIELD OF THE INVENTION

This invention relates to the repair and removal of erosion or impactdamage to elastomeric coatings on a curved substrate, particularly suchsurfaces as the leading edge of the airfoil which may take the form of awing, a rotor blade, a turbine blade, a propeller blade, a fan blade, anaircraft radome or antenna which have similar arcuate leading surfaces.The method is also useful for other flat and contoured surfaces.

BACKGROUND OF THE INVENTION

Elastomeric polymeric compositions are used to protect structures withforward facing surfaces, such as wings, rotor blades, propeller blades,fan blades, turbine blades, aircraft radome, and aircraft antennas.These structures can be severely damaged when used in their intendedoperational environments. The term “erosion damage” is a broad termencompassing damage caused by rain erosion, sand and dust erosion aswell as impact damages caused by stone, gravel or foreign objectsencountered typically in flight conditions.

Elastomeric erosion resistant coatings are supplied in the forms oftapes, sheets, molded boots and sprayable coatings. Currently availableelastomeric polyurethane coatings used in erosion protection applicationare highly sand erosion resistant, demonstrating higher sand erosionresistance than metal. However, elastomeric polyurethane coatings havelower rain erosion resistance than metal, usually exhibiting rainerosion damage in the form of deep pits, cracks, craters, and holes. Thesize, shape and location of the damage sites vary depending on thenature of the damage. The size and shape can vary from crack lines asthin as hair lines, pits about 1 millimeter or smaller in diameter,craters about 2 to 3 millimeters in diameter, or irregularly shapedholes wider than 1 centimeter across. The damage sites can existisolated and randomly distributed, or continuous across the forwardfacing surfaces.

When these erosion damages occur, it is extremely difficult to conductrepairs on the rain eroded polyurethane elastomers. The high sanderosion resistance makes it extremely difficult to remove the coatingsby hand sanding. For helicopters, removal of the current types oferosion protection coating by mechanical or chemical means requires theremoval of the rotor blades from the aircraft and typically removal bymachine sanding or other techniques. The reapplication of the tape,molded boot and sprayable coatings in the field is very labor intensiveand costly.

Another method to remove the damaged coating uses chemical strippers.This method also requires the removal of the rotor blades from theaircraft, as the open air will dry out the chemical stripper veryquickly. Another problem is that chemical stripping introduces hazardouschemicals into the operation. In addition, typical erosion resistantcoatings are used at a thickness equal or greater than 0.014″. Itusually takes overnight soaking to soften the coatings so that they canbe removed. There are also concerns that the stripper solution may swelland damage the composite structure under the erosion resistant coatings.For these reasons, it is not practical to do field repair with chemicalstripper.

Possible methods that could be used to repair the erosion damage involvebrushing on repair material and spraying on the repair materials.Neither of these methods is entirely satisfactory to fill in the cracks,holes of varying sizes and shapes on a curved surface, while stillmaintaining a smooth, aerodynamic surface at the end of the repairoperations. The extra layers simply follow the irregular contours of thedamaged surfaces interfering with aerodynamics of the airfoil. None ofthe methods employed to date have satisfactorily provided a method tofield repair a rotor blade which has erosion damage.

It is an object of this invention to provide a method to efficientlyfill in the pits, cracks, craters, and holes of varying sizes and shapeson a curved surface.

It is an object of this invention to provide a method to repair airfoilstructures such as the rotor blades that can be accomplished in thefield.

It is a further object of this invention to repair the rotor blade orother leading edge structure while the blades are still mounted on theaircraft or equipment,

It is an additional object to design an erosion protection system thatcan be removed and/or repaired in the field, without power tools orchemical strippers.

It is another objective of this invention to provide an erosionprotection coating system for airfoils and a repairable resin system forairfoils with contrasting colors to allow early detection of erosion,impact and other damages, and to allow fast repair to lengthen theservice life of the blades or structures.

SUMMARY OF THE INVENTION

One embodiment of the invention relates to a method of repairing anairfoil surface having a plurality of damage cavities caused by erosionor impact damage comprising filling said plurality of damage cavities insaid surface with a liquid repair material using a flexible applicatorcapable of conforming to the surface of said airfoil surface while beingdrawn lengthwise along the airfoil surface. This method may include thepreliminary steps of sanding the portion of said airfoil surfacecontaining said plurality of damage cavities with abrasive material andapplying an optional primer coat over sanded areas. The liquid repairmaterial is preferably formulated as an elastomeric basecoat and anerosion resistant topcoat may optionally be applied over the basecoat.The erosion resistant topcoat is preferably more sand erosion resistantthan the underlying elastomeric basecoat.

Another embodiment is directed to an elastomeric airfoil erosionprotection coating for visual detection of water and sand erosion damagecomprising having the elastomeric basecoat of a contrasting color to thecolor of the topcoat layer which is visible on the outer surface. Thissystem of contrasting colored coating layers provides visual detectionof any damage by detecting the appearance of the contrasting color theunderlying layers thereby indicating damage in the area. In anotherrelated aspect, there are three contrasting colored layers (a) a primerof a first color applied directly on a structural substrate of saidairfoil surrounding a leading edge of said airfoil; (b) a basecoat of asecond color applied over said primer; and (c) a topcoat of a thirdcolor on top of said basecoat, wherein said first color, second colorand third color are contrasting colors allowing visual detection ofdamage to said protection coating by visual inspection to detect theappearance of the second color of said basecoat or first color of saidprimer indicating damage in the area. The contrasting colored coatingsystem is used in a method of detecting damage to an airfoil erosionprotection coating allowing the slight damage to be repaired before theairfoil substrate is damaged, thereby prolonging service life. Therepair method described above is useful for such repair.

Still another embodiment is directed to a method of making an airfoilleading edge erosion protection coating capable of being fieldrepairable by hand sanding comprising applying to an airfoil substrate acoating system composed of a hand sandable basecoat and a topcoat, saidbasecoat being of lower sand erosion resistance than the topcoat andsaid basecoat constituting at least 50% of the total coating thickness.A related aspect relates to repairing said erosion protection coating bysanding the damage cavities until all irregular edges extending abovethe surface of said coating have been sanded until said edges are flushwith the surface; applying a repair basecoat to fill said plurality ofcavities to form filled cavities; and finally applying a repair topcoatlayer over the filled cavities.

Still another embodiment is a field repairable polymeric erosionprotection composition positioned on and adhered to a leading edgesurface of an airfoil comprising an elastomeric base composition loadedwith fillers sufficient to render the polymeric erosion protectioncomposition hand sandable, said base elastomeric base composition testedin accordance with ASTM D412-92 prior to incorporation of said fillershaving a minimum tensile strength of 1000 psi, an elongation at break ofat least 200%, and a Shore A hardness of less than 95 A.

A further embodiment is directed to a repairable elastomeric coating fora leading edge surface of an airfoil comprising (a) an elastomeric, handsandable basecoat disposed surrounding said leading edge surface havinga sand erosion rate above 0.020 grams/cm²; and (b) an elastomerictopcoat disposed on top of said elastomeric basecoat having a sanderosion rate below 0.020 grams/cm². Preferably the basecoat constitutesat least 50% of the total coating thickness.

A still further embodiment relates to a single layer erosion resistantelastomeric coating adhered to a leading edge surface of an airfoilcomprising a single layer of an elastomeric basecoat adhered on saidleading edge of said airfoil having a sand erosion rate above 0.020grams/cm² and a water erosion rate of greater than 100 minutes.

An embodiment utilizing applicators which are preformed to conform tothe cross sectional contour of the leading edge surface of an airfoilare useful on long uniform cross section leading edge application, andit utilizes an applicator comprising a body having an open interior areadefined by at least one interior wall, said wall having a wiping edge ata distal end thereof and a front edge generally opposite said wipingedge; said wiping edge defining a contour complimentary to a leadingedge surface of an airfoil; said front edge defining a contour shaped toform a pocket between said leading edge surface of said airfoil and saidinterior wall; and wherein during operation an elastomeric material isresident within said pocket so that as said applicator is drawn alongsaid leading edge surface of said airfoil said elastomeric material isdeposited on said leading edge surface of said airfoil and follows theshape defined thereby. This applicator may have a handle incorporatedtherein and a inlet device to allow for feeding in of the elastomericrepair materials by various supply methods including squeeze bottles,caulking gun-type devices and dispensing devices which meter and premixthe elastomeric repair material.

An additional embodiment relates to an airfoil repair kit comprising aflexible applicator capable of conforming to a leading edge surface ofan airfoil; and at least one an elastomeric, hand sandable repairmaterial along with optional kit components of sanding discs, a primer,an elastomeric basecoat, an elastomeric topcoat, brushes, and a sprayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is partial section of the leading edge portion of an airfoilstructure showing sand and water erosion damage.

FIG. 2 is a partial section of the leading edge portion of an airfoilstructure showing more severe sand and water erosion damage.

FIG. 3 is a partial section of the leading edge portion of an airfoilstructure used for laboratory testing coated with elastomeric erosioncoating.

FIG. 4 is a cross sectional schematic view of an airfoil shape withmajor airfoil or hydrofoil elements identified

FIG. 5 is a perspective view of a leading edge being repaired using aflexible applicator

FIG. 6 is partial section of the leading edge portion of an airfoilstructure showing sand and water erosion damage being repaired using aspecially formed flexible applicator.

FIG. 7 is partial section of the leading edge portion of an airfoilstructure showing sand and water erosion damage being repaired using aspecially formed flexible applicator with a handle

FIG. 8 is partial section of the leading edge portion of an airfoilstructure showing sand and water erosion damage being repaired using aspecially formed flexible applicator with an inlet for repair fluidbuilt in to the body of the flexible applicator.

FIG. 9 is a schematic representation of a method of testing sand erosionresistance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show a time lapse sequence of the erosion damageprogression on an airfoil shaped structure 10, the leading edge portion12 of which is shown in FIGS. 1 and 2 in sectional view. Rain erosionand impact damage (14 and 16) typically occurs at the front of theleading edge 12, while sand or solid particle erosion tends to focus onthe contour surfaces slightly away from the leading edges. Rain erosiontypically caused pits, craters, and holes, while sand erosion typicallyproduces uniformly matte surface appearance and very shallow erosionhole patterns. FIG. 2 illustrates a later stage of erosion damage usingthe same section from FIG. 1. In FIG. 2 the most severe erosion sites 20and 22 occurs when the surface is first eroded by sand or dustparticles, and then followed by rain erosion. Under this mixed sand/rainenvironment, the side surfaces region 24 surrounding the leading edgesare typically eroded into deep pits and craters very quickly.

Certain embodiments relate generally to the repair of an elastomericcoating 26 on a curved surface 28 of an airfoil shaped structure 30 asillustrated in FIGS. 1 and 2 as rotor blade 10. The elastomeric coating26 is defined as a flexible coating based on elastomeric polymercomposition. The coating may contain no filler or it may containfillers. The presence of filler may stiffen up the coatings to the pointof relatively little elastomeric physical character, but these filledcoatings are still regarded as “elastomeric coatings” for use in variousembodiments of this invention.

FIG. 3 shows the test airfoil 40 which is a mock-up of the partialairfoil leading edge section of an actual rotor used to simulate actualdamage from water and sand impingement in a controlled environment. Theelastomeric coating 42 is deposited on the underlying substrate 44surface area of the whole test airfoil. The leading edge 46 is the focalpoint for the impingement of water and sand during testing shown bydirectional arrow 48. All along the leading edge 44 and all the adjacentsurfaces represented by this test airfoil damage occurs by theappearance during testing of the erosion damage cavities shown in FIGS.1 and 2.

FIG. 4 illustrates by a cross-sectional diagrammatic representation ofthe convention structural portions of a typical airfoil 50 having aleading edge 52 and a trailing edge 54 with the oncoming wind directionshown as arrow 56, the angle of attack 58 is the angle between the winddirection and the chord 60′ of the airfoil 50 shown as a dashed line60′.

The wind carries sand and rain and debris into contact with the leadingedge 52 and impinges on its contoured surfaces 62′ and 64′ on eitherside of the leading edge. These leading edge areas are the test surfacessimulated by the test airfoil of FIG. 3. and are generally where thedamage occurs as best shown in the drawing representations in FIGS. 1and 2.

Terminology Definitions

The term “airfoil” as used throughout this specification is meant to bemore expansive than the conventional airfoil shaped structure in FIG. 4and will also encompass structures such as hydrofoils which have anaerodynamic shape that is somewhat different than FIG. 4 but aresimilarly subject to wind or water carried sand and debris. Suchincluded shapes are radomes shape which would have a leading edge in theform of a narrowed point rather than the leading edge which isgeometrically a line in FIG. 4 airfoil form. Aircraft antennae areshaped to allow smooth airflow around them and are considered within theterm “airfoil” as are other devices benefiting from the advancement ofthe embodiments such as windmill blades, turbine blades, runner blades,fan blades, compressor blades, propeller blades, vanes, stay vanes,hydroelectric turbines, marine propellers, hydro turbines, gas turbines,tide mills, windmills, compressors, pumps, blower, impellers,propellers, and many kinds of fans all of which have the common featureof having fluid passing by the surfaces which may carry damaging sandand debris.

The term “leading edge” will similarly be understood to have a broadermeaning than shown in FIG. 4 and should be defined as a narrowed surfacedesigned to encounter the wind or other fluid such as water. It is maybe an elongated narrowed edge in the case of a rotor blade, wing,antenna, windmill blade or a sharper edge surface as in a propellerblade or a forward wind encountering point area as in a radomes (whichmay have a blunt conical form or other generally rounded shape).

The term “elastomeric” as used herein generally is understood to be anyflexible material which has an ultimate elongation at break as measuredby ASTM D412-92 of at least 40% at break, preferably 80% and morepreferably 100%.

The terms “sprayable” or “spray-applied” or “sprayed” all are meant tomean materials that are coated and bonded onto a substrate, such as anairfoil, particularly the leading edge and surrounding areas using spraytechniques. This terminology distinguishes elastomeric materials thatmay be applied to the substrate as a premolded and/or preshaped boot ofelastomeric material, a preformed tape material which is adhered to theairfoil or a preformed sheet that may be bonded to the airfoil.

The term “hand sandable” is understood to mean a material whose surfaceis abraded away as loose debris within one minute of hand sanding. Thehand sanding is done on a properly supported 1.5″×3″ area of the testmaterial using moderate downward pressure with 80 grit aluminum oxidesandpaper. A “hand sandable” coating is characterized by the samplebeing able to be sanded by hand pressure into powder in less than 15seconds without gumming up or rolling up as an agglomerate. Excellentsandability preferably included one or more of the following additionalproperties: 1) sanding debris is coming off from the coating within 10seconds of sanding, 2) Low friction during sanding, 3) No heat or lowheat generation after one minute of hand sanding with normal effort, 4)Loose sanding debris in free flowing powder form, instead of gum up orrolled up agglomerate, 5) the amount of sanding residue on sanding discis equal to or less than the amount left on the coating after sanding.Using the Particle Erosion Test Apparatus, we have found that materialswith sand erosion rate of less than 0.020 grams are difficult to sand byhand. To be hand sandable by the definition of this invention, thematerial should have a sand erosion rate of higher than 0.020 grams,preferably higher than 0.030 grams and 0.040 grams. Materials havingsand erosion rates between 0.02 and 0.03 sometimes exhibit slightly moredifficult hand sanding characteristics.

It has been found to be very difficult to repair the pits, craters, andholes scattered throughout the leading edge of a helicopter rotor bladeor other leading edge surfaces. Common repair techniques of using aputty knife and putty-like solid repair resin do not work well in thisapplication. The damage sites can be too small for a material with puttyconsistency to flow in. The putty knife cannot repair a curved surfaceeither.

A helicopter rotor blade or other leading edge structures are welldefined aerodynamically shaped surface. The airfoil shape of the rotorblade is characterized with a very sharp curve at the leading edge. Arepair resin must have the proper viscosity so that it does not run ordrip from the sharp curve during the repair procedure. Any repair to theblade surface must minimize the distortion of the aerodynamic contour.

The difficulty of repairing a damaged elastomeric surface has limitedthe total service life of an elastomeric erosion protection system on ahelicopter rotor blade. Currently, elastomeric molded boot and selfadhesive polyurethane tape are used to protect the blades. Once thedamage occurs on surface and in the body of the elastomeric materials,the self adhesive elastomeric tapes may encounter sudden catastrophicadhesion loss and fly away from the rotor blades during flight, whichbecome a safety concern for aerodynamic balance of the rotors. Thealternative existing elastomeric covering of a rotor takes the form of apreformed, molded boot that is adhesively bonded to the blade substrate.The elastomeric boot is usually left to erode until not usable and thenreplaced. Replacing the tape and the boot are both very labor intensiveoperations, involving the removal of the rotor blade from thehelicopter, stripping off all old coatings by a variety of methods,applying some replacement elastomeric materials and then carefullyconducting weight balancing of the blade after the repair procedure. Ifthe field unit is not equipped to do the repair, the entire rotor blademust be sent back to a depot facility to do the repair and overhaul. Thetransportation of the rotor blades is costly and time consuming.

Another deficiency of the current erosion protection methods is the lackof an early erosion indicator that enables the user to take preventiveaction to stop the erosion going all the way through the elastomericcoating and ultimately into the substrate. The commercial erosionresistant sprayable coatings use one color gloss or matte color schemes.If a basecoat and a matte topcoat are used, the prior art coatingsystems typically use a gloss or semi-gloss basecoat, and a mattetopcoat, both of the same or very similar colors. In these systems, eventhough the underlying primer or adhesive of different texture ordifferent colors may be utilized, the total system does not providesufficient warning for the users to take preventive actions when thereis slight damage to the elastomeric coating. Once the underlying primeror adhesive is exposed, the elastomeric protective coating is damaged tothe point of not being serviceable any longer. This inability to detectearly and slight damage shortens the service life of the rotor bladesand other airfoil-type structures with leading edges, such as radomesand antenna structures on the aircraft. Because the prior artelastomeric erosion protection materials typically erode to thesubstrate with deep cratering and pitting, the damage usually reach thesubstrate before any corrective actions can be taken. This can bedetrimental to a composite structure as the underlying composite layersof the rotor or airfoil can be punctured through by rain erosion in avery short period of time.

Still another deficiency of the existing erosion protection systems isthe difficulty of coating removal from the substrate. Coating removal isan essential part of a successful field repair procedure. An elastomericerosion resistant coating by its nature is very difficult to remove. Thecommon methods use a solvent based stripper to soak through the coatingto soften or dissolve the primer. Typical primers suitable for suchprocedure include polyvinyl butyral based wash primers. While theprocedure works well to remove the erosion resistant coatings, the useof excessive amount of hazardous solvents is not desirable. In addition,it takes a long time, typically overnight soaking, to soften or dissolvethe primer. In a military or other emergency operations, the helicoptercannot be out of service for many hours, waiting for this lengthy repairprocedure.

Many advantages can be achieved by the repair methods and procedures inthe embodiments described hereafter.

The field repair of the cavities caused by rain erosion or impact damageon the curved surfaces of an airfoil structure can be accomplished withthe use of a flexible airfoil contour applicator also called a FlexibleApplicator (FA) as described more fully below. Additional steps in therepair of rotor blade damage may include one or more of the followingsteps: 1) Surface preparation including sanding, 2) Application ofprimer, 3) Application of basecoat, and 4) application of topcoat.

1. Surface Preparation Step

On a rain or impact damaged surface, there are holes and cut surfaces,with some remaining debris hanging around the wells of the craters, pitsand holes. This “raised” debris must be removed or smoothed tocorrespond with the surrounding contoured surface. Exacto knives can beused, but are discouraged due to the risk of damage to the compositesubstrates. We have found that a pair of scissors, most preferablycurved scissors, can be used to trim off the raised debris. The curvedscissors has a curvature that can touch the damage sites at proper angleto trim off the debris. This is especially helpful in the surfacepreparation of sand erosion resistant elastomeric erosion protectioncoatings, since they are very difficult to smooth out with abrasivesanding. For example, elastomeric polyurethane coatings containing nofiller or low concentration of fillers tend to “smear” or “gum up” whenabrasive sanding is used. These coatings will be extremely tiring for aworker or soldier to sand the large rotor blade in the repair procedure.

Hand Sandable Embodiment

To be practically repairable in the field, the new erosion protectionsystem of this embodiment should preferably be sandable by hand in thefield, on the aircraft, without the need to remove the rotor blade fromthe aircraft. In one preferred embodiment, the coating is made to behand sandable on purpose. This is a significant departure from thecurrently employed erosion protection materials. The conventionalerosion protection method strives to make the elastomeric coatings orresins as erosion resistant as possible, thus making the unfilled orlightly filled/pigmented elastomer extremely difficult to remove bysanding when repair is needed. These materials are not “hand sandable”as defined below. This embodiment discloses the opposite concept in thedesign of the erosion protection system. In this embodiment, additionalfillers are added to decrease the sanding resistance of the basecoat onpurpose, and in many applications where sand impingement is encountered,a thin layer of sand erosion resistant topcoat is used on top of thesandable basecoat to form the total erosion protection system. In thispreferred embodiment, the thin layer of the topcoat and the thick layerof the basecoat can be sanded with the use of proper grade of sandingmedium, yet still achieve high erosion protection against rain and sanderosion. By using this new concept with the added early erosionmulti-color warning indicator that will be described in detail below, afield repairable, renewable erosion protection system for protection ofthe leading edges of airfoils is achieved.

On a helicopter rotor blade or other airfoil-type leading edges havingvery well defined aerodynamic surfaces, conduct of an electrically orpneumatically powered sanding operation is a dangerous procedure as oversanding can easily damage the composite honeycomb structure underneaththe composite skin. Electrical or pneumatic power sanding may be used ina depot environment where experienced personnel routinely perform thesanding procedure, but are not practical for a field repair environmentwhere inexperienced personnel are handing the sanding tasks undernon-ideal working conditions. It is preferred to use hand sandingbecause the human hand can sense the contour of the substrate anddynamically adjust the degree of sanding force against the coating foroptimum removal without damage to the substrate. If sanding discs areused for hand sanding, self-adhesive palm sized sanding discs arepreferred. Those with self-stick adhesive that can be held securelyattached to the palm of the hand are especially useful. Grit sizesbetween 40 to 200 grits may be used, with 80-120 grits especiallypreferred. We have found, quite surprisingly, a stiff sanding disc witha center insert, originally designed for metal grinding at high speed,works especially well as an optimal hand sanding disc to sandelastomeric erosion resistant materials. The small rigid center insert,as seen on commercial sanding discs such as 3M Roloc TSM 361F discs,allows the finger to press the sanding disc hard against the elastomericcoatings to effectively remove the coating. A sanding block may be usedfor less contoured surfaces when the technicians are highly skilled andwell supervised.

Examples of some of the preferred tools and accessories for use in therepair kit which will be later described in detail may include: Curvedscissors, Roloc sanding discs, and self-stick sanding discs. The sandingof the damaged area may create loose coating debris and powders. Theseloose powders and debris must be removed from the work surface beforethe repair resins can be applied. To remove the loose debris andpowders, it has been found that different solvents have differentcleaning power. A good cleaning solvent does not attack or soften theerosion protection elastomers, but is able to pick up the loose powderseffectively. Slower evaporating solvent is preferred as the field repairis conducted outdoor in open air. We have found that non-polar solventsuch as toluene and xylene are especially preferred for in the repairprocedure. Lint free wipers are preferred for use with the cleaningsolvent in this procedure.

2. Application of Primer

If the erosion damage reaches the substrate, an adhesion promotingrepair primer is usually required. Afterwards, a basecoat is applied tofill in the cavities, with the aid of the flexible applicator using theapplication method of deforming the flexible applicator to conform tothe contour of the airfoil allowing the basecoat repair resin to bespread with the flexible applicator into the cavities without leavingrepair resin on the undamaged portions of the airfoil surfaces. Once thebasecoat is hardened then it is followed by application of the topcoat.

When the primer is eroded and the substrate is exposed, the repairprimer, which may be an epoxy primer, must be used with great care andprecautions to prevent it from being inadvertently deposited on top ofthe intact original elastomeric coating. It has been experimentallyfound that if spots or areas of the epoxy primer are left on top of anelastomeric polyurethane erosion resistant coating, the epoxy primerwill cause early erosion initiation, probably due to the stiff, highmodulus nature of the epoxy base of the primer which is markedlydifferent from the lower modulus of the basecoat causing stressed todevelop at the interface which cause cracks and premature failure of thebasecoat integrity. Therefore, the primer must be applied only to theexposed substrate areas at the bottom of the cavities without any primerbeing overlapped onto the undamaged surrounding elastomeric coatingsurface.

Because of the typical small size of the rain erosion induced damagecavity, depositing the proper amount of primer is a significantchallenge which requires skill and practice to achieve. Most paintbrushes used in any normal painting jobs are too big for this procedure.Practice of this embodiment preferably includes the use of micro-sizedtips or brushes for the repair of erosion protection system. Especiallypreferred are the tips or brushes that can control the deposit size toabout 1.0 mm, 2.0 mm and 3.0 mm in diameter. These dot-placement brushesare very useful in priming the craters, pits, cracks, and holes. Theycan also be used to apply the primer to an area larger than craters,pits and small holes. For erosion damages that have enlarged to asomewhat bigger area, small width bristle brush can be used. Examples ofsuitable applicators for applying the primer are Microtip, Microbrushand Ultrabrush manufactured by Microbrush International, Wisconsin, USA.

The repair primer may be formulated from suitable known primer basesincluding but not limited to epoxy, polyvinyl butyral, polyurethane orother polymer system with good adhesion to the substrate. It ispreferred to have a fast drying and fast curing primer so that theerosion resistant coatings can be applied on top of the primer withinshort time such as one to two hours. When priming, with the specialmicro-sized brushes, the superfine round tip Microbrush is used todeposit micro dots into the small pits and craters. A larger brush isused for spreading the primer onto bigger areas, preferably using about3/16″ wide strokes to “paint” larger areas with primer. When the primerbecomes tack free or cures to proper stage (depending upon the primerbase system), it is ready to be coated with the basecoat.

3. Repair of the Basecoat

The basecoat is formulated to cure in a relatively thick film or layerand be flexible. The Repair Basecoat may have a pot life of about 30minutes to four hours after mixing. This range of pot life provide areasonable work time for the repair procedure. Longer or shorter potlife may be used depending on the environment and work schedule. Thecoating gets thicker as time goes on and becomes very viscous, but stillspreadable. This dynamic change of viscosity can be used to goodadvantage to do the repair. When the viscosity is still low (coatingstill has thin consistency for about the first 30 minutes), the repairresin can be used to deposit a thin layer onto the damaged areas. Thefluid coating will spread into the micro-pits and craters and seal theprimed surfaces. As the viscosity increases, the repair resin can beused to build up the coating thickness faster as it has less tendency toflow on its own.

For isolated small pits and craters, the Microbrush and the Ultrabrushcan be used to deposit the basecoat into the small openings. In contrastto the primer application, the basecoat repair can use heavy, thickdeposits. In this case, the Microbrush can deposit a thick layer ofbasecoat upon one single contact with the substrate without spreading.

On a rotor blade, turbine blade, propeller blade and other fan blades,the thickness of the blade may change along its length, from the inboardsection to the outboard section. The blade may also have a twist alongthe surface. To apply the thick basecoat efficiently in one application,this embodiment discloses the use of a flexible applicator for thispurpose. The flexible applicator is bendable along the curvature of theleading edge surface. The size of the flexible applicator can be as bigas the area to be repaired. For helicopter rotor blades, the rainerosion damages usually focus around 2 inches (5 cm) on both sides ofthe leading edge of the blade, while combined sand and rain erosiondamages typically occurs within 8 inches (20 cm) on the sides of theleading edge of the blade. Therefore, a flexible applicator withcoverage of 8 inches or less on both sides of the rotor blade will besufficient. Larger size or smaller sizes can be used depending on theactual contour and dimension of the blades.

The flexible applicator can also be used to apply the coating onto theflat surface of the blade. In this case, the edge of the applicator isused like a flat scrapper to smooth out the coating on a flat surface.

The flexible applicator can be made of a semi-rigid, bendable material,which can be metal, plastic, or rubber. It needs to be rigid enough tohold its shape, but flexible enough to bend along a continuouslychanging curvature. Flexible semi-rigid plastic sheets are preferred.Especially preferred are semi-rigid, flexible plastic sheets with highsolvent resistance and good release properties. High densitypolyethylene and polypropylene sheets are particularly preferred

It has been found that one of the simpler forms of the flexibleapplicator is a 0.010″ (0.25 mm) thick high density polyethylene sheetwhich has the proper combination of being flexible enough to bend alongand conform to the curved surface, while still being rigid enough tohold its shape to apply and shape the coating along the curvature. Bothpolyethylene and polypropylene have excellent solvent resistance andrelease properties.

FIG. 5 is shown as a photograph because it provides the bestvisualization of the application technique of this embodiment. Theairfoil chosen for illustration is a helicopter rotor blade 60 having aleading edge 62 which has damage cavities 64 in its contoured surfaces.The flexible applicator 66 is made of a 0.010″ (0.25 mm) thick highdensity polyethylene sheet. The flexible applicator edge 68 forms acontinuous line contact with the contoured surface of the leading edgeusing downward pressure indicated by the force vector arrow 70. At thesame time the force 70 is applied in the direction of the surface, theflexible applicator is drawn in a direction 72 that is parallel to theleading edge 62. The basecoat repair material (not visible in this view)is under the curved surface of the flexible applicator in a rolling bankof material that is moved ahead of the flexible applicator edge 68 asthe applicator is smoothly drawn in the direction 72. The basecoatrepair material completely fills the damage cavities 64 as the rollingbank of repair material passes over the cavities. The applicator edge'scontinuous line contact with the contoured surface of the leading edgedoes not deposit significant amounts of repair material anywhere exceptin the cavities 64.

Types of Flexible Applicators

It has been experimentally determined that the curved surface or airfoilrepair embodiment is enhanced by the use of a “Flexible Applicator”(FA). In the simplest embodiment the flexible applicator is a flexibleplastic sheet. In using the simplest, flat sheet-type of flexibleapplicator, the coating can be deposited onto the substrate, or onto theapplicator, or both surfaces. The flexible applicator is then “dragged”along, or “pulled” along the curvature, generally in the longitudinaldirection or long dimension of the airfoil, rotor or blade. It has beenfound that applying a sufficient amount of repair coating material onboth the applicator and substrate surfaces gives a helpful “lubricatingeffect.” This technique allows the flexible applicator to “glide” easilyalong the curvature of the airfoil surface.

In practicing this embodiment, it has been found that it is preferred touse moderate force to pull the flexible applicator tightly against thesubstrate surface. By maintaining the force on the applicator, goodcontrol of the coating thickness is maintained and there is notexcessive build up of the repair coating on an undamaged curved surfaceof the airfoil being repaired. It has been determined that keeping theflexible applicator wet and lubricated is very beneficial in doing therepair procedure. When the resin becomes dry or the repair resin isexhausted, the repair resin gets sticky and does not break cleanly. Theapplicator embodiments of FIG. 8 deal effectively with this byincorporating the feature of continuous feed of resin into the surfacecontact region under the flexible applicator. This combination ofdesirable characteristics of the flexible applicator technique assuresthat the aerodynamic characteristics of the airfoil being repaired isnot altered by the repair. One important characteristic of thisembodiment is the unique ability to fill in the irregularly shaped andsized pits, cracks, craters, and holes in one application on each curvedsurface. In comparison to conventional method of filling each erosiondamage site individually, this method of application results in greatsavings of time and labor.

The dimension of the flexible applicator needs to be wider and longerthan the size of the damage cavities. The dimension of the flexibleapplicator is such that the semi-rigid, semi-flexible applicator is ableto maintain the outside contour of the original curved surface, so thatit spread the liquid coating to a thickness not thicker than theoriginal outside contour of the airfoil.

This is very important to an aerodynamically sensitive structure likerotor blade, radome, antenna, fan blade, turbine blade, etc.

The repair resin may be applied onto the leading edge surface first, andthen the flexible plastic sheet is positioned over the leading edge andpulled along its surface. Or the repair resin may be applied onto theplastic sheet and then it is pulled over the damaged surface area. Orthe repair resin may be applied to both the leading edge and the plasticsheet, and then the plastic sheet is pulled along the leading edge tothin out the resin and squeeze the resin into the holes and craters.

Alternative Flexible Applicator Embodiments

Using the same flexible/conformable scraper blade concept, various handapplicator tools can be designed to fit well with the leading edgestructure of various shape and sizes. Such flexible applicators arewithin the contemplation of this embodiment and various thicknesses,shapes and materials are contemplated as suitable for a flexibleapplicator so long as they are capable of following the contour of thecurved leading edge surfaces. FIG. 6 shows a preformed flexibleapplicator 70 which is made of suitable plastic or metal and formed intoa flexible yet permanent shape of the exact cross sectional outercontour 72 of the cross section of the airfoil 74 being repaired. Theedge 76 of the flexible applicator 70 is contoured to be complementaryto the outer contour 72 and will function to move a rolling bank ofrepair material 78 ahead of it as it is moved parallel to and along theleading edge 80 of the airfoil 74 as shown by arrow 82. The wider, openend 84 accommodates the volume the rolling bank of repair material 78which completely fills in any cavity 86 in the surface 88 of the airfoil74 being repaired as the flexible applicator is advanced along theleading edge of the airfoil. The shape of this end of the applicator isnot limited by any consideration except leaving enough volume to be ableto replenish the repair material, ergonomics of the operator and factorsrelating to how it is manufactured. The material used for this flexibleapplicator should have elastic memory or sufficient stiffness after itis formed to the proper curvature of edge 76 to be able to wipe anyexcess repair material 78 ahead of the edge 76 in a wiping or squeegeeaction. It is understood that the edge 76 could have a arc that isslightly smaller that the leading edge to assure that there is pressureexerted by the applicator in the direction of the surface by the elasticmemory or general stiffness of the material. This embodiment is usefulfor portions of the airfoil structures that a leading edge which hasrelatively uniform cross section to the over a substantial length of theairfoil. This embodiment is very useful in that it does not rely ondelicate adjustments of downward pressure by the operator to maintain agood squeegee action. The tapered outer tip of the rotor blade or wingsurface could be finished off with a planar flexible applicator 66 asillustrated in FIG. 5.

FIG. 7 shows a preformed flexible applicator 90′ which is substantiallysimilar in most regards to the applicator of FIG. 6 although thereference characters are not repeated. This embodiment includes a handle92′ enables better control by the operator of the orientation, directionand downward force applied toward the airfoil leading edge 94′. Thedirectional arrow 96′ illustrates again the direction the applicatorwith the handle is moved. This handle may enable the operator to useonly one hand to handle the applicator, allowing the other hand to befree to feed in repair material 98′ as needed during repair operations.It also is essentially self tensioning through the natural downwardforce applied by the operator while keeping it in position against theairfoil leading edge. The handle 92′ may be incorporated into thestructure of the applicator by any conventional means, including beingintegrally molded into the applicator body or attached by any suitablemethod to the applicator body. The shape of the handle can be anysuitable shape.

FIG. 8 shows another embodiment of a flexible applicator 100 whichincludes a fluid connector 102 which connects to the portion of theapplicator that holds the repair material 104. The fluid connector hasadapter 106 to attach to any suitable reservoir of repair material (notshown). This reservoir may be a simple bottle which can manuallydispense the repair material 108 as shown by the directional arrow 108into the fluid connector by squeezing a flexible bottle. It may be acaulking gun configured to feed the repair material into the fluidconnector. The dispensing device may be a triggered device for meteringeither a single fluid or multiple fluids. In an embodiment where therepair material is of the type that has a two part composition, such asa part a base component and a part b curative material, the dispensingdevise may be of the type that can meter the proper ratio of part a andpart b and optionally also mix those parts together in a mixingapparatus prior to dispensing the repair material through the fluidconnector into the working interior volume of the flexible applicator102 where the rolling bank of repair material is positioned during useof the device.

The embodiments of FIGS. 7 and 8 can also be consolidated such that thehandle 92 of FIG. 7 and the fluid connector 102 of FIG. 8 are in aunitary dual function structure. Of course separate handle and fluidconnector are also easily designed to fit onto the specialized flexibleapplicator which can be easily manipulated by a single person. For useas the repair basecoat, the coatings without filler should beelastomeric enough to be erosion resistant to rain or sand Additionalfillers may be added to increase the sand erosion rate.

The repair resin/coating may be 100% solid without solvent or it maycontain diluents such as solvent or water. The repair resin may bereactive or non-reactive (fully pre-reacted). It may contain some or allof the following ingredients: resins, curing agents, fillers, fibers,fabrics, viscosity modifier, pigments, hydrolysis stabilizers, adhesionpromoters, coupling agents, UV stabilizers, defoamers, wetting agents,etc. The repair resin/coating may be as fluid as a brushable coating upto as viscous as a flowable caulking compound.

For use as sandable, erosion resistant coating, the coating is made froma highly flexible coating composition with additional fillers added at asufficient level to allow for particulate removal the top surface of thepolymer during sanding. The organic polymers suitable for forming thehand sandable coatings can comprise polyacetals, polyureas,polyurethanes, polyolefins, polyacrylics, polycarbonates, polyalkyds,polystyrenes, polyesters, polyamides, polyaramides, polyamideimides,polyarylates, polyarylsulfones, polyethersulfones, polyphenylenesulfides, polysulfones, polyimides, polyetherimides,polytetrafluoroethylenes, polyetherketones, polyether etherketones,polyether ketone ketones, polybenzoxazoles, polyoxadiazoles,polybenzothiazinophenothiazines, polybenzothiazoles,polypyrazinoquinoxalines, polypyromellitimides, polyquinoxalines,polybenzimidazoles, polyoxindoles, polyoxoisoindolines,polydioxoisoindolines, polytriazines, polypyridazines, polypiperazines,polypyridines, polypiperidines, polytriazoles, polypyrazoles,polycarboranes, polyoxabicyclononanes, polydibenzofurans,polyphthalides, polyacetals, polyanhydrides, polyvinyl ethers, polyvinylthioethers, polyvinyl alcohols, polyvinyl ketones, polyvinyl halides,polyvinyl nitriles, polyvinyl esters, polysulfonates, polysulfides,polythioesters, polysulfones, polysulfonamides, polyureas,polyphosphazenes, polysilazanes, polyolefins, polysiloxanes,fluoropolymers, polybutadienes, polyisoprenes, or a combinationcomprising at least one of the foregoing organic polymers. Exemplaryorganic polymers are polyurethanes, polyureas and fluoropolymers. It isdesirable for the polyurethane, the polyurea and the fluoropolymers tobe an elastomer. The aforementioned organic polymers listed above can beblended and/or copolymerized with the polyurethane or polyurea ifdesired. The base elastomers can be fully reacted such as water basedpolyurethane, fully reacted thermoplastic elastomers such aspolyurethane, TPR (Thermoplastic rubber), EPDM rubber, nitrile rubber,chlorinated rubber, butyl rubber, SBR (styrene butadiene) rubber,fluoroelastomer, silicone rubber, natural rubber, etc. The mostpreferred elastomer is polyurethane and fluoroelastomers

The isocyanates in the polyurethane elastomers can be aromatic oraliphatic. Useful aromatic diisocyanates can include, for example,2,4-toluene diisocyanate and 2,6-toluene diisocyanate (each generallyreferred to as TDI); mixtures of the two TDI isomers;4,4′-diisocyanatodiphenylmethane (MDI); p-phenylene diisocyanate (PPDI);diphenyl-4,4′-diisocyanate; dibenzyl-4,4′-diisocyanate;stilbene-4,4′-diisocyanate; benzophenone-4,4′-diisocyanate; 1,3- and1,4-xylene diisocyanates; or the like, or a combination comprising atleast one of the foregoing aromatic isocyanates.

Useful aliphatic diisocyanates can include, for example,1,6-hexamethylene diisocyanate (HDI); 1,3-cyclohexyl diisocyanate;1,4-cyclohexyl diisocyanate (CHDI); the saturated diphenylmethanediisocyanate known as H(12)MDI; isophorone diisocyanate (IPDI); or thelike; or a combination comprising at least one of the foregoingisocyanates.

Other exemplary polyisocyanates include hexamethylene diisocyanate(HDI), 2,2,4- and/or 2,4,4-trimethyl-1,6-hexamethylene diisocyanate,dodecamethylene diisocyanate, 1,4-diisocyanatocyclohexane,1-isocyanato-3,3,5-trimethy-5-isocyanatomethylcyclohexane (IPDI), 2,4′-and/or 4,4′-diisocyanato-dicyclohexyl methane, 2,4- and/or4,4′-diisocyanato-diphenyl methane and mixtures of these isomers withtheir higher homologues which are obtained by the phosgenation ofaniline/formaldehyde condensates, 2,4-and/or 2,6-diisocyanatotoluene andany mixtures of these compounds.

In one embodiment, derivatives of these monomeric polyisocyanates can beused. These derivatives include polyisocyanates containing biuret groupsas described, for example, in U.S. Pat. No. 3,124,605, U.S. Pat. No.3,201,372 and DE-OS 1,101,394; polyisocyanates containing isocyanurategroups as described, for example, in U.S. Pat. No. 3,001,973, DE-PS1,022,789, 1,222,067 and 1,027,394 and DE-OS 1,929,034 and 2,004,048;polyisocyanates containing urethane groups as described, for example, inDE-OS 953,012, BE-PS 752,261 and U.S. Pat. Nos. 3,394,164 and 3,644,457;polyisocyanates containing carbodiimide groups as described in DE-PS1,092,007, U.S. Pat. No. 3,152,162 and DE-OS 2,504,400, 2,537,685 and2,552,350; and polyisocyanates containing allophanate groups asdescribed, for example, in GB-PS 994,890, BE-PS 761,626 and NL-OS7,102,524. In another embodiment,N,N′,N″-tris-(6-isocyanatohexyl)-biuret and mixtures thereof with itshigher homologues and N,N′,N″-tris-(6-isocyanatohexyl)-isocyanurate andmixtures thereof with its higher homologues containing more than oneisocyanurate ring can be used.

Examples of suitable polyols are polyester polyols, polycaprolactonepolyols, polyether polyols, polyhydroxy polycarbonates, polyhydroxypolyacetals, polyhydroxy polyacrylates, polyhydroxy polyester amides andpolyhydroxy polythioethers. Exemplary polyols are polyester polyols,polyether polyols, polyesters derived from lactones (e.g.,ε-caprolactone or ω-hydroxycaproic acid), or a combination comprising atleast one of the foregoing polyols.

Exemplary isocyanate prepolymers are TDI-ether, TDI-ester, TDI-lactone,MDI-ether, MDI-ester, H12MDI-ether, H12MDI-ester and similar prepolymersmade from HDI, IPDI and PPDI. The isocyanate prepolymers with low freeisocyanate monomers are preferred.

The coating composition also comprises an optional curing agent.Examples of suitable curing agents are aromatic amines that can be usedas curing agents are phenylene diamine,4,4′methylene-bis-(2-chloroaniline), 4,4′methylenedianiline (MDA),4,4′methylenebis(2,6-diethylaniline),4,4′methylenebis(2,6-dimethylaniline),4,4′methylenebis(2-isopropyl-6-methylaniline),4,4′methylenebis(2ethyl-6-methylaniline),4,4′methylenebis(2,6-isopropylaniline),4,4′methylenebis(3-chloro-2,6-diethylaniline) (MCDEA),1,3-propanediolbis(4-aminobenzoate), diethyltoluenediamine (DETDA),dimethylthiotoluenediamine; or the like; or a combination comprising atleast one of the foregoing aromatic amines. Polyaspartic esters may beused. Polyol curatives are polyester polyols, polycaprolactone polyols,polyether polyols, polyhydroxy polycarbonates, polyhydroxy polyacetals,polyhydroxy polyacrylates, polyhydroxy polyester amides and polyhydroxypolythioethers. Exemplary polyols are polyester polyols, polyetherpolyols, polyesters derived from lactones (e.g., ε-caprolactone orω-hydroxycaproic acid), or a combination comprising at least one of theforegoing polyols. Imines are useful curatives, including aldimines,ketimines, multifunctional imines.

Atmospheric moisture may serve to cure solely or may catalyze thereaction between the polyurethane and the curing agent. This is referredto as moisture cure. For aqueous coatings, polyurethane dispersions canbe used with or without curing agents. The crosslinking of aqueouspolyurethane dispersions may be accomplished by the use of isocyanates,epoxy, or aziridines functional materials.

Other additives useful in the coating compositions include levelingagents, adhesion promoters, coupling agents, defoamers, hydrolysisstabilizers, UV stabilizers, pigments, dispersants, curing accelerators,diluents, or combinations thereof.

In order to exhibit high erosion resistance with the fillers, thebasecoat preferably utilizes a coating composition in which theelastomeric base of the repair coating prior to the addition of anyfillers has been determined to preferably have a minimum tensilestrength of 1000 psi, an elongation at break of higher than 100%, and aShore A hardness of less than 95 A, more preferred is 200% elongationand most preferred 350% elongation. These properties are generallytested according to ASTM D412-92 or D2370 if a film coating is beingtested. Exemplary elastomeric bases along with specialized testing andtest methods are as disclosed in U.S. patent application Ser. No.11/136,827, filed May 24, 2005, which is incorporated herein byreference in its entirety.

The fillers that may be used to render the elastomeric basehand-sandable and will also increase the sand erosion rate for therepair basecoat layer include, but are not limited to, the followinglist:

silicates (such as talc, clays, (montmorillonite) feldspar, mica,calcium silicate, calcium metasilicate, sodium aluminosilicate, sodiumsilicate), metal sulfates (such as calcium sulfate, barium sulfate,sodium sulfate, aluminum sodium sulfate, aluminum sulfate), gypsum,aluminum trihydrate, metal oxides (such as calcium oxide (lime),aluminum oxide, titanium dioxide, iron oxide, tin oxide) and metalsulfites, metal powders, metal flakes, metal fibers, milled metalfibers, metal nitrides, graphite, carbon nanotubes, carbon fibers andmilled carbon fibers, silica (such as quartz, glass beads, glass bubblesand glass fibers), metal-coated glass spheres, metal-coated hollowspheres, buckyballs, electroactive polymers, antimony-doped tin oxide,carbon blacks, coke, micro-balloons, and oxides, borides, carbides,nitrides and silicates from the group of compounds containing boron,aluminum, silicon, titanium, tungsten, and zirconium compounds.

Examples of organic based fillers can be used include thermoplasticpowdery material such as polycarbonate, polyetherimide, polyester,polyethylene, polysulfone, polystyrene, acrylonitrile-butadiene-styreneblock copolymer, Teflon, fluoropolymers, polypropylene, acetal polymers,polyvinyl chloride, polyurethanes, nylon and combinations thereof. Ingeneral, some useful thermoplastic polymers are those having a highmelting temperature or good heat resistance properties. There areseveral ways to form a thermoplastic abrasive particle known in the art.

The useful fillers have a hardness greater than that of the materialforming the continuous phase of the coating. The particle size of thefillers may be from nano-sized to 200 microns, or preferably less than100 microns. The filler content in the hand sandable coating, based onthe total solid weight, can range from 10% by weight to 90%, dependingon the interaction of the fillers and the base elastomers. Preferred is20% to 80% by weight and more preferred is 30% to 70% by weight.

The surface gloss of the basecoat may be gloss, semi-gloss or matte. Insome applications, the repair basecoat may be used without additionaltopcoat. For those applications that require a different surface glossor different functional surface properties, another topcoat layer may beapplied. The topcoat may be used to change the surface gloss, surfacetexture, or surface properties, such as antistatic or electricalconductivity. As earlier described, the topcoat may also be formulatedto provide higher erosion (sand and water) resistance and applied over abasecoat. In the preferred embodiment, the sandable erosion resistantbasecoat layer constitutes at least 50% of the total coating thickness.

If the coating is 100% solid, one application with this procedure willfill in the cavities of the damage sites to their full height. If thecoating contains solvent, the dry coating thickness depends on the drysolid content of the coating. In this case, a second application may beapplied to build up the dry film thickness at the damage sites. Eventhough the evaporation of the solvent left very slight indentations atthe damage sites, one application of the basecoat with the uniqueflexible applicator was able to repair the rotor blade quickly and thehelicopter was able to continue flying in a short time period with nodetrimental aerodynamic effects on the rotor blade.

The basecoat described here is used to fill in the erosion and impactdamage sites and cavities.

We have found a very efficient method to repair the deep craters, pitsand holes formed by erosion and impact damages. First, the repair resinis formulated so that there is a somewhat greater degree of “body” to itat the time of repair. The repair resin can be thixotropic, shearthinning, or simply having at least moderate viscosity. “Moderateviscosity” means that the repair resin can be brush applied and does notflow away from the applied surface. The repair resin can preferably bereactive, in which case the viscosity increases with time after thecomponents are mixed together. The repair resin can also be nonreactive,being a fully reacted resin dissolved in solvent or water.

In practicing this invention, the repair resin/coating may containspecial effect fillers, additives, fibers, fabrics to provide specialfunctions and properties. If the added filler reduces the erosionresistance of the resin/coating, another layer of the topcoat withhigher sand or rain erosion resistance can be applied on top of therepair resin/coating. In this case, the repair procedure comprises theapplication of primer (optional), the basecoat and the topcoat. Thetopcoat may be formulated to provide the desired color, gloss anderosion resistance, but in general not hand sandable, by the definitionof this invention. The invention may also be applied to single ormulti-layered coating systems.

Hand Sandable Elastomers Testing techniques

One method to determine whether a coating is hand sandable is to use ahand sanding test. Another method is to use a mechanical particulateerosion test or a Taber Abrasion apparatus and then correlate to theease of the hand sanding.

Hand Sanding Test

The coating materials are either spray coated onto the substrate orglued to the substrate with a double faced permanent pressure sensitiveadhesive. A 3″ diameter sanding disc, 3M Roloc TSM 361F, with 80 gritaluminum oxide abrasive, is to be used as the sanding medium. The discis stiff with metal hub at the center. The disc is bent on both sidewith fingers, and the middle section is pressed down against theelastomeric coating surface by using two central fingers. Usingmoderately firm pressure, the sanding is done with a timer clock for oneminute. The sanding was focused in a small area about 1.5″×3″ indimension. The weights before and after the hand sanding were recorded.

COMPARATIVE EXAMPLE 1

Caapcoat Black B-274, a sprayable rain erosion resistant coatingmanufactured by Caap, Inc. was sanded as in the above procedure. Thecoating felt gummy, with a lot of resistance to sanding. The sandingdisc got hot after about 15 seconds of hand sanding. Only trace amountof sanding powder/debris was obtained. The arm used in the hand sandingfelt sore and tired after 40 seconds. The weight loss after one minuteof sanding was 0.029 grams

COMPARATIVE EXAMPLE 2

Caapcoat FP-200, a gray sprayable rain erosion resistant basecoat usedin a basecoat-topcoat FP-250 coating system, was hand sanded. Thecoating felt gummy, with a lot of friction. The hand got tired afterabout 40 seconds. Low sanding dust was observed. There was some heatbuilt up around 30 seconds. The weight loss after one minute was 0.040grams.

COMPARATIVE EXAMPLE 3

Caapcoat White, a gloss white sprayable rain erosion resistant coating,was sanded. Results were similar to Comparative Example 2. The weightloss after one minute was 0.022 grams.

COMPARATIVE EXAMPLE 4

Caapcoat Fluoroelastomer V, a gray sprayable elastomeric rain erosioncoating, was sanded. The film used for the sand test was 0.002″ thickdue to the low solid content of the coating. The coating was sanded. Thefilm ripped through easily due to low film thickness. However, poorsandability with very low sanding dust was observed. The weight lossafter one minute, including the ripped pieces, was 0.050 grams.

COMPARATIVE EXAMPLE 5

Chemglaze M331, a gloss black sprayable rain erosion resistant coatingmanufactured by Lord Corporation, was sanded. The coating produced verylow sanding dust after one minute. The hands get tired after about 50seconds. The weight loss was 0.024 grams after one minute.

COMPARATIVE EXAMPLE 6

A piece cut from a Task L-101 molded boot manufactured by Task Inc. wassanded. The sanding disc got very hot in about 7 seconds. The sandinghad to be continued by switching fingers to be comfortable. The weightloss was 0.062 grams after one minute.

COMPARATIVE EXAMPLE 7

3M 8545 tape, a black molded erosion resistant polyurethane sheetmanufactured by 3M Company, was sanded. The sanding disc got very hot in15 seconds. The material felt gummy, with trace of sanding dust rolledup together in lumpy form. The weight loss was 0.028 grams after oneminute.

COMPARATIVE EXAMPLE 8

3M 8667 tape, a black molded erosion resistant polyurethane tape withpressure sensitive adhesive backing, was sanded. There was a lot offriction. The sanding disc got very hot in 15 seconds. The trace sandingdust rolled up into small lumps. The weight loss was 0.018 grams afterone minute.

As seen in the above Comparative Examples, a person trying to sand asmall 1.5″×3″ area for one minute using the current commercial erosionresistant coating could not remove much material, at the same time, theperson felt tired, exhausted and also encountered uncomfortable heatgenerated in very short period of hand sanding. Thus when trying toutilize the materials of the Comparative Examples 1-8, it would not bepractical or even possible to conduct a field repair of a rotor blade,which may measure about 20 feet long.

The hand sanding properties are determined by the total filler loading.As the filler loading increases, the polymeric film on top of theelastomer can be broken away and form loose debris, thereby making thehand sanding easier to perform. Because each filler has its own densityand surface properties, the interaction of filler and the base elastomervaries and can be determined by experimental trials.

In contrast with the above Comparative Examples, a good hand sandablecoating produced loose debris in powder form, with substantial amount ofdebris left on the coating surface after sanding, instead of beingtrapped inside the abrasive particles on the sand paper. In similarprocedure by the same person using the same technique, the weight lossof the hand sandable coating is higher than 0.080 gram, preferablyhigher than 0.100 grams, and even more preferably higher than 0.150grams.

FIG. 9 illustrates an example of the mechanical sand erosion apparatusas practiced in the Particle Erosion Test Apparatus, operated by theUniversity of Dayton Research Institute, Dayton, Ohio. In this test,particles 90 are accelerated in a small diameter (approximately0.25-inch) high-speed gas jet 92 and directed onto a test specimen 94 asillustrated in FIG. 9 Since the diameter of the jet is smaller than thetest specimen area, the specimen holder and jet are articulated so thatthe test specimen 96 is moved through the jet in a uniform manner. Thisarticulation provides a uniform particle loading (particle massintercepted per unit surface area) over square area of approximately 316cm2 (i.e., 7.0-inch square). The inner 6 inch square is considered validtest area. For the sand erosion test using a flat 1″×1″ specimen, thenet sand erosion exposure area is a circle of 2.0 centimeter.

Compressed air 98 provides the transport gas stream with regulators andpressure transducers to measure and control the pressure at the nozzleinlet. Particles are metered into the transport gas stream from apressurized screw feeder system. Since the screw feeder provides a veryaccurate and uniform particle flow, the particle mass applied to thespecimen is determined by the run time based on prior calibration of thescrew feeder.

Velocity is determined as a function of the nozzle inlet pressure byprior calibration. Thus, for a given test, a specific test velocity canbe selected from this velocity versus pressure calibration. Particlesize, velocity and impact angle 97 can be controlled independently. Thisprovides an excellent capability to parametrically evaluate the responseof critical materials and coatings to solid particle impact effects.Materials from such components as rotorcraft blade coatings, leadingedges, windscreens, radomes, paints, and any special coatings can beevaluated in a well-controlled laboratory environment under realisticparticle impact conditions.

The Particle Erosion Test Facility differs from the real flightenvironment in that the specimen is stationary and the particle field ismoving at the specified impact velocity. Whereas the key parameters inthe flight environment are the static cloud mass concentration (mass orvolume of particles per unit volume) and velocity, in the particleerosion facility the key parameters are the particle mass loading andvelocity. The relationship between the mass loading in the testfacility, and dust cloud concentration, impact velocity and time in theflight environment is as follows:

Mass Load=Concentration*speed*time(*unit conversion factors).

Specimen size of 1 inch square is used to determine the sand erosionrate. The sand erosion was conducted with dry silica sand that have beensieved to 177-250 microns (um), Sand is sieved from F-series ungroundsilica from U.S. Silica at a mean particle stream velocity of 353 milesper hour, using an impact angle of 30 degrees. The mass of impingingparticles is set at 10 grams per square centimeter.

In practicing this invention in sandy environments, a layer of high sanderosion resistance elastomer is used on top of the sandable basecoatlayer. To maintain the sandability, it is preferred to let the sandablebasecoat occupy at least 50% of the total coating thickness. In general,it is preferred to use 0.004″ or thinner layer of the topcoat. In thisembodiment, the sand erosion will erode the top layer, and then thebasecoat and primer. When the basecoat is exposed, the erosion damage isfirst covered with a renewable sand erosion resistant coating. When thebasecoat is eroded, it is easily sanded down and repaired with theprocedure disclosed in this invention.

In one embodiment, the basecoat is configured to have a sand erosionrate (mass weight loss) of greater than 0.024 grams when testedaccording to the Particle Erosion Test Apparatus under 353 mph, 30degree impact angle, 1″×1″ specimen size, with 177-250 micron sandparticles, more preferably greater than 0.030 grams, It is even morepreferred to have the basecoat configured to have sand erosion rate ofhigher than 0.040 grams for better hand sandability.

In another embodiment, the basecoat is configured to have a sand erosionrate (mass weight loss) of greater than 0.024 grams, and at the sametime contains a topcoat layer of having a sand erosion rate of less than0.020 grams. It is more preferred to have a basecoat layer with sanderosion rate of higher than 0.040 grams, and a topcoat layer with a sanderosion rate of less than 0.015 grams. It is even more preferred to havea basecoat with sand erosion rate of greater than 0.050 grams.

For use in the water environment without sand erosion concerns, thebasecoat layer containing filler that retains good rain erosionresistance can be used alone, forming a sandable rain erosion protectionlayer.

Application of the Repair Topcoat

For minor damage situations where the erosion has only removed thetopcoat and exposed the underlying basecoat, these areas need onlytopcoat repair. In addition, pits and craters smaller than 1/16″ canalso be repaired by repairing the topcoat only. Slightly damagedsurfaces can be wiped clean with solvents such as xylene, toluene, butylacetate or MEK (methyl ethyl ketone).

Early Erosion Warning System Embodiment

This embodiment discloses the use of contrasting color in forming theairfoil erosion protection system. The coating system may comprise of aprimer of color A, a basecoat of color B, and an optional topcoat ofcolor C. The colors of A, B, and C are formulated to provide a colorcontrast so that when the erosion reaches at each layer, it provides avisual warning and indication of the need for repair. The use of primeris optional, as some basecoat resin systems may possess sufficientadhesion that no primer is needed. In some cases, the coating system maycontain only primer and basecoat, or in others only basecoat andtopcoat.

In one embodiment, the basecoat is formulated to be in grayish color toprovide contrast to the matte black topcoat. This serves as an EarlyWarning Indicator for erosion damage. The service life the rotor and itselastomeric protective coating can be greatly increased if routinerepair procedures incorporate regular inspection for any visualindication of damage and if any is found, four to six repair layers ofmatte topcoat are sprayed whenever the gray basecoat is exposed toprevent any further erosion of the basecoat. The matte black topcoat isdesigned for use as a regular maintenance touch-up coating. It is to beused whenever the gray basecoat becomes visible.

In routine use, a repair sprayable topcoat is applied whenever thetopcoat is eroded away and the gray basecoat is shown. The topcoat issprayed while the rotor blade is still on the aircraft, in the field.According to the repair method embodiment, the spraying of the topcoatis used as the first line of defense against erosion damage.

The topcoat may be applied by brushing, dipping or spraying. If anunderlying repair basecoat has been applied, the heavy thickness of thebasecoat makes it preferred to allow time for the solvent to flash offfrom the basecoat before the topcoat is applied. Depending onapplication environment, one to two hours waiting time is generallysufficient. To obtain the best matte appearance, spraying is thepreferred application method.

Spraying of the coating can be accomplished by any of the known sprayingmethods, including, but not limited to trigger sprayer, air poweredpressure sprayer, propellant-powered sprayer, aerosol sprayer, pumpsprayer, etc. For field repairs away from a pressurized air supplysource, a small disposable hand trigger sprayer or aerosol propellantpowered sprayer is especially preferred. An example of suitablepropellant powered sprayer is the Preval Paint Sprayer (Spray Gun). ThePreval Paint Sprayer includes a propellant-filled power unit for thesprayer and a container for the paint.

Typically a single spraying pass of the matte topcoat deposit about0.0005″ (0.5 mils) of dried topcoat. Although it may vary with severityof damage it has been found that 4-6 spraying layers (0.002-0.003″) areused to maintain the erosion resistance of the coated blades afterrepair. The topcoat can be sprayed as many coats as needed. To maximizethe sand erosion resistance, a topcoat with low filler content, highsand erosion resistance is preferred. After repair, a rotor blade withrenewed erosion resistance is placed back to service.

Helicopter Rotor Blade Field Repair Example

An experimental sprayable coating system was prepared for application onhelicopter rotor blades. The coating system contains a green epoxyprimer, a grayish black filled basecoat, and a matte black topcoat. Thecolors of Green (primer)/Grayish Black (basecoat)/Matte Black (topcoat)forms the Early Erosion Warning Indicator system.

The basecoat was an ambient temperature curable coating systemcomprising polyurethane with high levels fillers. When hand sanded forone minute as described in the earlier hand sanding test, the basecoathas a weight loss of 0.204 grams and generated a lot of loose powder in5-10 seconds. There was no heat built up during the one minute handsanding. When this basecoat was sand tested at UDRI, the 1″×1″ UDRI sandtest erosion rate was 0.058 grams.

The topcoat is an ambient temperature curable polyurethane. The topcoatis not hand sandable, with a weight loss of 0.031 grams when subjectedto hand sanding test for one minute. The topcoat also has very good rainerosion resistance and sand erosion resistance.

A rotor blade was spray coated with a 0.001″ thick of green epoxyprimer, a 0.017″ thick grayish black, filled elastomeric polyurethanebasecoat, and a 0.002″ thick polyurethane matte black topcoat layer,containing carbon black filler.

During a flight test, the rotor blades encountered severe sand erosionand stone and gravel impact damage during operation in a desertenvironment. The damage appeared as numerous pits, craters, cracks andholes. The damaged blade surface had a mixture of visible colorsindicating damage to the different colored layers. The colors werebright gray (substrate metal), bright green (primer), grayish black(basecoat), and matte black (topcoat). The colors were of good contrastand the various degree of erosion/impact damages were easily visibleduring an inspection.

The blade was sanded with 80 grit sand paper. A solvent wipe using alint free wiper cloth was used to clean up the loose sanding dust. ARepair Primer was mixed and brushed onto the exposed metal sites with asmall brush trimmed down to ⅛″ tip. Any excess primer was wiped cleanwith the solvent wipe. After curing for about one hour, a RepairBasecoat was mixed. The urethane Repair Basecoat had a brushableviscosity as mixed, gradually increasing to pasty consistency, andeventually to gel-like state. The viscosity increase was used to depositthe liquid repair resin onto the various sizes of damage cavities.

Various techniques and tools were tried for use on the airfoil surfaceto fill in the numerous tiny holes and craters on the leading edge. Itwas later found that by bending a semi-flexible 0.010″ thick highdensity polyethylene sheet, the sheet can function as an effectiveFlexible Applicator (FA) sheet in a special way along the leading edge,the numerous pits, craters, and holes were filled in one simpleoperation. We found that by applying the brushable repair basecoat ontoboth the leading edge and the FA, a lubricating effect happened and therepair efficiency increased. Normally, one pass of the FA was able tofill in most of the holes and cracks. For some areas with deeper damage,an additional pass was applied after the first coat dried.

The repaired leading edge was left to dry and cure for about one andhalf hours. The repair topcoat was mixed and sprayed 4 to 6 passes usinga disposable Prevail spray gun. After overnight cure, the repaired rotorblades looked very good and the helicopter was ready to fly again. Thisrepair procedure was conducted in the open airfield, with the rotorblades still mounted on the aircraft, using a staging platform to liftthe workers up and down.

The repaired aircraft continued to fly for very long time with regulartouch-up repair procedure of this invention. The erosion resistantcoating system, the repair resin and the repair procedure of thisinvention in combination provided more than 30 times improvement overthe polyurethane tape products.

This example demonstrates the successful combination use of an earlyerosion warning system, the primer repair procedure, the basecoat repairprocedure and the topcoat repair procedure.

Example of a Repair Kit

A Repair Kit for Airfoil Elastomeric Leading Edges is a very useful andnovel combination of the particular materials described herein packagedin convenient carrying package for easy transport by mobile maintenanceteams. The Repair Kit will preferably contain at least the followingitems: 1) A Flexible Applicator and 2) Repair Material which includes atleast one of the following: repair primer, repair basecoat or repairtopcoat, depending upon the type of damage encountered. For applicationsnot needing a topcoat, only the basecoat and optionally a primer isincluded. For applications without need of basecoat repair, only therepair topcoat needs to be included. For applications both basecoat andtopcoat, both repair materials are include as well as the optionalprimer. The Kit may also optionally include any of the following items:special lint free wiper towels, curved trim scissors, disposable gloves,respirators, special selected sanding discs, solvents such as xylene,squirt bottle, disposable ½″ brushes, disposable Microbrushes,disposable Ultrabrushes. The Kit may be housed in any convenientpackage, for example, paper carton, plastic storage container and/or aplastic carry-on container for easy transport in military mission use.

Alternate Embodiments

While solvent stripping is not the preferred method for field repair,the repair methods disclosed in the embodiments herein are compatiblewith the solvent stripping coating removal method in the proper workenvironment. For example, solvent stripping in combination with therepair method embodiments can be practiced satisfactorily in a depotfacility. For certain substrates such as radomes, sand blasting or otherspecialized media blasting techniques may be used to remove damagedmaterial prior to repair as described in various embodiments herein.

Although the embodiments set out herein disclose the methods andmaterials for use in the airfoil repair procedures, it is readilyapparent that the methods and materials embodied can be applied to newerosion protection systems for use on various airfoil leading edgesurfaces which benefit from elastomeric erosion protection.

While this patent application describes the repair and the removal ofsprayable coating, the same principles apply to molding resins, moldedboots, tape, brushable coating, putties, and caulking compounds. Theseapplications should be treated as part of this invention.

What is claimed is:
 1. A method of repairing an airfoil surface havingan elastomeric protective coating adhered to a portion of said airfoilsurface having a plurality of damage cavities caused by sand and watererosion or impact damage comprising: filling said plurality of damagecavities in said elastomeric protective coating with a liquid repairmaterial of elastomeric polyurethane, polyurea or fluoroelastomer havingviscosity between a brushable coating up to the viscosity of a flowablecaulking compound sufficient to fill said plurality of damage cavitiesto form filled cavities using a flexible planar applicator capable ofconforming to said airfoil surface while being drawn lengthwise alongthe airfoil surface.
 2. The method of claim 1 wherein the contactbetween the airfoil surface and said flexible planar applicator ismaintained by applying pressure in the direction of the airfoil surfaceto fill the damage cavities with the liquid repair material.
 3. Themethod of claim 1 further comprising applying a primer coat in theplurality of damage cavities.
 4. The method of claim 1 wherein saidelastomeric polyurethane, polyurea and fluorolastomer composition havinga minimum tensile strength of 1000 psi, an elongation at break of atleast 100%, and a Shore A hardness of less than 95 A.
 5. The method ofclaim 1 wherein said flexible planar applicator is a flat sheet ofelastomeric or thermoplastic material of selected thickness to allow itto be deformable around an arcuate surface and stiff enough to spreadviscous material smoothly ahead of it when drawn through the viscousmaterial.
 6. The method of claim 1 further comprising a preliminary stepof applying one or more layers of primer coat over any airfoil surfaceexposed within said damage cavities using a fine brush capable ofcontrolling the primer coat deposition to about 1.0 mm to about 3.0 mmin diameter.
 7. The method of claim 1 further comprising a preliminarystep of applying one or more coats of said elastomeric polyurethane,polyurea or fluoroelastomer to damage cavities using a fine brushcapable of controlling the deposition within a range of about 1.0 mm toabout 3.0 mm in diameter prior to said filling step.